The goal of this project is to explore the idea that some forms of primary lymphedema are caused specifically by defective valve formation resulting from mutations in Connexin (Cx) genes. Lymphatic (Ly) system function is essential for normal tissue fluid balance, immune function, and absorption and transport of dietary fat, and valves ensure that lymph moves forward rather than abnormally collecting or refluxing. Defects of the Ly system, including those that may involve defective or incompetent valves, lead to a number of congenital and acquired disorders. Lymphedema, in particular, is a potentially debilitating disorder in which interstitial fluid abnormally accumulates and is estimated to affect as many as 140-250 million people worldwide. Despite progress in identifying key genes involved in Ly development, the molecular mechanisms controlling the morphogenesis of Ly valves remain poorly understood. Thus, increased knowledge about the molecular pathways functioning during Ly valve development provides an excellent opportunity for new approaches to prevent or treat lymphatic disorders such as lymphedema. Recently, a new key player in valve development was discovered: Cxs - a family of related proteins which assemble into gap junction intercellular channels, structures that allow for the direct transfer of small molecules between adjacent cells. Previous vascular work focused on the role of Cxs in the development, remodeling, and physiology of arteries and arterioles. However, we have discovered that three Cxs (Cx37, Cx43, and Cx47) are expressed in the Ly endothelial cells of developing Ly vessels and that these proteins become progressively enriched at Ly valves (and venous valves). Using global Cx knockout mice, Cx37 and Cx43 were shown to be critical for Ly valve formation. Although Cx47 was shown to be mutated in some families with primary lymphedema, nothing is known about the role of Cx47 in Ly development or function. Based on the location of the Cx47 missense mutations in highly conserved Cx extracellular loops, we propose that the mutations are dominant negative in nature and result in primary lymphedema through a combined inhibition of wild-type Cx47 and Cx43 (which colocalize at valves), leading to defective valve development. In this proposal, we will compare the effects, in mouse models, of a Cx47 null mutation and a putative Cx47 dominant negative mutation that causes lymphedema in humans. The effects of these mutations on Ly valve formation, Ly function, and development of lymphedema will be assessed in the mice. In addition, the effects of a combined null mutation of Cx47 and Cx43 will be determined as another test of the theory that both Cx47 and Cx43 must be inhibited for a severe disease phenotype. The mouse lines generated in this project will provide valuable new animal models for the study of primary lymphedema and allow for a better understanding of how Ly dysfunction initiates or contributes to disease. In addition, the mouse models will be useful for future studies aimed at identifying the local physiological conditions that contribute to secondary lymphedema.